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ID:26406426
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时间:2018-11-26
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1、1.Obviously,theprincipalTrade-off(平衡,折中考虑)betweenserialandparalleltransmission(串行和并行传输)isspeedversussimplicity.Datatransmission(数据传输)canbeaccomplishedmuchmorequicklyusingparalleltransmission;however,paralleltransmissionrequiresmorelinesbetweensourceanddestination(在源和目的地之间)
2、.Asageneralrule,paralleltransmissionisusedforshort-distancecommunications(短距离通信),andwithinacomputer,andserialtransmissionisusedforlong-distancecommunications(而串行传输则常用于远距离通信)。2.Webeginourhistoricalsketchin1969withtheRS-232-Cstandardfortheserialportofcomputerdevice(计算机的串口),T
3、hisstandardisforlowbitratetransmissions(upto38kbps)overshortdistances(lessthan30m)(它是短距离(<30米)传输的低速率(38kbps)传输标准).Serialtransmissionprecedesonecharacteratatime(每次一个符号).Thecomputerdevicesencodeeachcharacter.intosevenbits,towhichtheycanaddaparitybitforerrordetection(可以添加一个奇偶
4、校验比特来检错),andsuccessivecharacters(连续字符)areseparatedbysometimeinterval(时间间隔).Whenthereceiverdetectsthebeginningofanewcharacter(当接收器检测到一个新字符的开始时),itstartsaclockthattimesthesubsequentbits.Bothbitrateanddistancemustbekeptsmall,becausetransmissionstakeplaceoveruntwistedwires(非双绞
5、线),whichcanintroduceerrorsduetocross-talk.Cross-talkbecomesmoresevereastherateandthedistanceincrease(随着速率和距离的增加).3-1.Linearpredictivecoders,(LPCs线性预测编码器)belongtothetimedomain(时域)classofvocoders.Thisclassofvocodersattemptstoextractthesignificantfeaturesofspeechfromthetimewa
6、veform(从语音信号的时间波形中提取一些重要的特征).ThoughLPCcodersarecomputationallyintensive.theyarebyfarthemostpopularamongtheglassoflowbitratevocoders(低速率声码器).WithLPC,itispossibletotransmitgoodqualityvoiceat4.8kbpsandpoorerqualityvoiceatevenlowerrates(可以用4.8kbps的速率传输质量较好的语音,还可以在更低的速率下传输质量差一些
7、的语音)3-2.Thelinearpredictivecodingsystemmodelsthevocaltractasanallpolefilter(全级点线性滤波器)withatransferfunctiondescribedby…WhereGisagain(增益)ofthefilterandzrepresentsaunitdelayoperation(单位时延操作).Theexcitation(激励)tothisfilteriseitherapulseatthepitchfrequency(与对应音律的频率相同的脉冲)orrandom
8、whitenoise(随机白噪声)dependingonwhetherthespeechsegmentisvoicedorunvoiced.Thecoefficients(系数)
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